Scala Cookbook

10.13. Transforming One Collection to Another with for/yield

Problem

You want to create a new collection from an existing collection by transforming the elements with an algorithm.

Solution

Use the for/yield construct and your algorithm to create the new collection. For instance, starting with a basic collection:

scala> val a = Array(1, 2, 3, 4, 5)
a: Array[Int] = Array(1, 2, 3, 4, 5)

You can create a copy of that collection by just “yielding” each element (with no algorithm):

scala> for (e <- a) yield e
res0: Array[Int] = Array(1, 2, 3, 4, 5)

You can create a new collection where each element is twice the value of the original:

scala> for (e <- a) yield e * 2
res1: Array[Int] = Array(2, 4, 6, 8, 10)

You can determine the modulus of each element:

scala> for (e <- a) yield e % 2
res2: Array[Int] = Array(1, 0, 1, 0, 1)

This example converts a list of strings to uppercase:

scala> val fruits = Vector("apple", "banana", "lime", "orange")
fruits: Vector[String] = Vector(apple, banana, lime, orange)

scala> val ucFruits = for (e <- fruits) yield e.toUpperCase
ucFruits: Vector[String] = Vector(APPLE, BANANA, LIME, ORANGE)

Your algorithm can return whatever collection is needed. This approach converts the original collection into a sequence of Tuple2 elements:

scala> for (i <- 0 until fruits.length) yield (i, fruits(i))
res0: scala.collection.immutable.IndexedSeq[(Int, String)] =
  Vector((0,apple), (1,banana), (2,lime), (3,orange))

This algorithm yields a sequence of Tuple2 elements that contains each original ...